1. Field of the Invention
The present invention relates to a tool holder used in a honing operation for grinding and finishing an inner surface of a hole formed in a workpiece with high precision. Specifically, the invention relates to a tool holder which allows a general-purpose machine tool to carry out multiple honing operations.
2. Related Art
Conventionally, a tool holder is used for a honing operation. The tool holder includes, on a leading end portion thereof, grinding shoes with their associated slender grindstones. Hereinafter, such tool holder is referred to as a honing holder. The honing holder, generally, is mounted on a main spindle of a honing machine. In a state where a grindstone is in surface contact with an inner surface of the hole of a workpiece, the honing holder is rotated and simultaneously reciprocated in an axial direction thereof by the honing machine, so as to minutely cut or abrasively finish the inner surface of the hole of the workpiece.
In such honing operation, in order to cool heat generated by grinding and also to wash away honed dust and grindstone grinding dust to thereby facilitate the honing operation, generally, a coolant (a cooling solution) is continuously sprayed onto a grinding portion.
Further, in order to be able to press the grindstones against the inner surfaces of the holes of the workpiece, generally, there is employed a structure in which a push rod including a taper portion (which is hereinafter referred to as a taper cone) is moved in the axial direction to push out the respective grinding shoes in the radial direction, thereby expanding all of the grindstones uniformly in a radial direction. That is, suppose a circle which has a radius constituted of a length extending from the axis of the honing holder to the grindstone, in the case that the respective grinding shoes are pushed out in the radial direction due to the movement of the taper cone in the axial direction, the diameter of this circle (which is hereinafter referred to as a grinding diameter) is expanded. Such expansion of the grinding diameter is generally realized by adjusting the pressure force of the taper cone using an oil pressure control function of the honing machine.
Recently, there has been made a request that such honing operation is carried out using a general-purpose machine tool not a special machine tool, that is, a honing machine. However, since the general-purpose machine tool, in many cases, does not have an oil pressure control function, in order to meet the above request, as a method for adjusting the pressure force of the taper cone, there is necessary a different method from the oil pressure control method. JP-B-07-004759 discloses an adjusting method using a coolant supply mechanism provided in the general-purpose machine tool. In the method, the pressure force of the taper cone is adjusted by controlling a supply pressure of a coolant.
However, since a coolant supply pump of the general-purpose machine tool is normally operated with a given pressure. Therefore, if the supply pressure of the coolant is changed, there can be raised a trouble in the coolant supply mechanism. Thus, the coolant supply mechanism of the general-purpose machine tool must be presupposed that the supply pressure of the coolant is constant or that, even if variable, it can be switched only roughly in several stages by opening and closing a valve.
Therefore, even when the method of JP-B-07-004759 is applied to a honing operation using the general-purpose machine tool under the above presupposition, it is impossible to finely adjust the pressure force of the taper cone. As a result, a grinding diameter to be expanded can not be varied so that a given fixed diameter can only be selected.
This raises the following problem. That is, for example, if multiple honing operations are carried out in stages from a rough finishing operation to a precision finishing, the grinding diameter should be changed whenever the respective honing operations are carried out, or, even when the grinding diameter remains the same, the proper surface pressures of the grindstones with respect to the hole inner surfaces of the workpiece should be varied. This makes it necessary to prepare a special honing holder for each of the multiple honing operations. This raises a problem that a cost of the tool holder is expensive. Also, it becomes necessary to replace a honing holder before and after each honing operation. This raises a problem that an operator must bear an excessive operation burden.